Hi Wizard,
The inlet water into the DI enters around the outside of the cap, travels down through the resin and into the bottom of the riser (the central column) and rises up (see?) to the top and exits through the centre of the cap.
Take the cap off, connect a water supply to one of the holes and see which bit of the cap the water comes through, if it's the outside you're on the right track, if it comes down from the middle you need to feed into the other hole.